Electrolytic recording medium



United States Patent Office 3,539,457 ELECTROLYTIC RECORDING MEDIUMRobert Doughty Richards, Summit, N.J., and Alan Howard Vincent, Anerley,London, England, assignors to Muirhead Instruments, Inc., Mountainside,N.J., a corporation of New Jersey No Drawing. Filed Apr. 23, 1968, Ser.No. 723,568 Int. Cl. B21h 1/20 U.S. Cl. 2042 8 Claims ABSTRACT OF THEDISCLOSURE An electrolytic recording medium for marking with a silveranode, the medium impregnated with calcium or strontium formaldehydesulfoxylate.

This invention relates to the art of electrolytic recording, and inparticular to an improved electrolytic recording medium or paper.

Recording paper of the electrolytic type is generally marked by passingthe paper between a positive eroding metal anode electrode and anegative non-eroding cathode electrode. The paper is impregnated with anelectrolytically conducting solution containing a marking agent. When avoltage is applied between the electrodes and current is passed throughthe recording paper, metal ions are introduced into the paper from theanode and reacted with one or more of the ingredients of the paperimpregnant to form a colored mark on the recording paper.

In order to be acceptable for facsimile recording purposes, therecording paper preferably should be white initially and should have theability to produce marks having a density which is at least close inlinear relationship to the current passed through the paper. The densityof the marked area should be variable uniformly in a range from whitefor zero current flow through grays for currents of intermediatemagnitude to black for currents of maximum value. The mark preferablyshould be black so as to provide a good contrast with the whitebackground.

There should also be a minimum of bleeding or fringing of the mark so asto provide good resolution. The unmarked paper while stored in moistcondition in sealed containers for extended periods of time prior to useshould remain stable without chemical decomposition of its ingredientswhich would cause discoloration of the paper or affect thereproducibility of results without recorder adjustment. Further, themarked recording paper should not become discolored, give off unpleasantor other odors, or be subject to the transfer of the mark to adjacentmaterials. The marked recording paper should be insensitive to lighteven after being repeatedly run through any of the conventionalduplicating machines involving exposure to ultraviolet or other stronglight sources.

Heretofore successful recording papers have been provided employingpyrocatechin, also called catechol, as the marking compound. Whilegenerally satisfactory some difficulty has been had with the catecholpapers because of the tendency of catechol to diffuse or transfer to andto discolor adjacent sheets of paper in contact with or in closeproximity thereto. Catechol vapors also may cause discoloration of thewalls and other surfaces near the recorder during the recordingoperation.

3,539,457 Patented Nov. 10, 1970 Prior to the development of thecatechol papers, attempts were made to provide papers utilizing a silverelectrode to supply the metal ions which were reacted with a reducingagent to form a mark. Such papers are disclosed in Elsey Pat. 2,063,992.Elsey teaches the use of formaldehyde to reduce silver ions in anelectrolytic recording paper so as to produce a black mark. It isbelieved that the mark formed on the Elsey marking medium diffusessomewhat even though reagents are included to reduce this tendency.Further, because the formaldehyde used by Elsey is volatile theimpregnated paper is unstable and thus is commercially impractical.

The next development in the art is the recording medium disclosed inLieblich Pat. 3,332,857. In this patent Lieblich discloses the use offormaldehyde in the form of sulfoxylates and also discloses otheraldehyde addition salts. The disclosures of this patent are limited tothe use of the aldehyde salts of the metals of Group 1A of the PeriodicTable of Elements, together with zinc or cadmium. In his disclosures theinventor makes much of the concept of combining formaldehyde in asulfoxylate or bisulphite compound.

The next development in the prior art is that shown in Barnes et al.Pat. 3,349,013 in which the aldehyde addition salts of Leiblich areimproved by the addition of certain stabilizers.

In marking the recording media of this art silver ions are released froma silver metal anode by the passage of an electric current. The silverions enter the paper and are rapidly reduced by formaldehyde to form ablack colloidal precipitate marking the paper.

During storage of these papers the recording characteristicsdeteriorate. The response as measured by the intensity of a markproduced by a particular applied voltage is not constant but variesaccording to the age of the paper. This disadvantage limits extensiveuse of the paper in the important area of newspaper photograph receptionand even in the less critical field of data reception the papers areconsidered unusable after storage for a few months.

It would be expected that the prior art paper deterioration is caused byoxidation of the formaldehyde. However, unexpectedly, we have found thatthe deterioration is due to the production of sulfites by decompositionof the addition salts.

The effect of sulfite is to cause polarization of the silver anode whichprevents silver ions from entering the paper. This effect can bedemonstrated by the addition of sodium sulfite to an otherwise effectivepaper which causes it to immediately behave as a paper in which theformaldehyde compound has suffered decomposition. Normal depolarizingagents are not effective to overcome this deleterious effect.

In accordance with the invention we find that the use of calcium orstrontium addition salts or mixtures thereof overcomes thisdisadvantage. Decomposition of the formaldehyde addition compound is notprevented but the sulfite compound which is formed removes itself byprecipitation. The solubility of the sulfites of these metals issufficiently low so as to prevent appreciable sulfite polarization ofthe anode.

Other objects and advantages of the invention will be apparent from thefollowing description.

Suitable formulations for impregnation into square 3 feet of recordingpaper stock or other porous support are as follows:

EXAMPLE 1 Grams Calcium formaldehyde sulfoxylate 75 Sodium nitrate 110Calcium hydroxide 2 Water 1000 EXAMPLE 2 Grams Strontium formaldehydesulfoxylate 69 Strontium nitrate 95 Strontium hydroxide 1 Water 1000With both of the formulations of the examples satisfactory marks wereobtained using silver anodes with currents and potentials of commercialpracticability such as is customarily used with the Lieblich and Barneset a]. recording media.

In the examples the concentration of the reducing agent has been set asapproximately 7 percent by weight. However, it is to be understood thatthe amount of reducing agent may be varied from a minimal amount up tothe solubility limit thereof. Further, various combinations of thealdehyde addition salts may be used rather than a single salt.

Typical concentrations of reducing agent may vary between 0.1 and 2percent by weight of available formaldehyde. These concentrations areselected as being commensurate with conventional recording speeds andmarking apparatus sensitivity.

In Example 1 sodium nitrate is used as electrolyte and in Example 2strontium nitrate is used as electrolyte. Other suitable electrolyteswhich may be used are the alkali metal nitrates, calcium nitrate, andammonium nitrate. The concentration of the electrolyte may be varied asdesired for purposes of varying the electrical conductivity as is knownin the art. Such variations may be from minimal amounts to thesolubility limits of the respective salts. Combinations of various saltsmay be made as desired.

It is desirable to provide a pH in the alkaline range because of thegreater stability of these reducing agents in a basic medium. The pH mayrange between 7 and 14. The preferred range of pH is about 7.0 to 11.0inasmuch as the paper or other support is generally more resistane todeterioration in this range. Alkali hydroxides, buffers or both may beused to obtain the desired pH value in this range. Acarbonate-bicarbonate buffer is preferred because it provides a goodmedium for the reducing action of the aldehyde addition salts. Theadvantage of using a buffer is that it will tend to prevent a drift inthe pH of the impregnant over a period of time. Suitable alternativebuffering agents are monohydrogen phosphate-db hydrogen phosphate andother buffers known in the art.

The eroding anode electrode may be made of silver, silver alloyed withcopper and/ or zinc, or any other silver alloy, copper, or any othermetal which provides an ion which may be reduced to give a colored mark.It will be noted, for example, that silver produces a good black mark,copper a green mark, and iron a light yellow mark. Silver or silveralloys are preferred anode electrodes because of the high opticaldensity of the mark produced and because of its stable image. Silver, inthe production of univalent silver ions, requires less energy thancommon metal electrodes which produce multivalent ions. The use ofsilver thus generally results in the production of good density markswith a small expenditure of power. Silver alloy electrodes are availablewhich provide increased 4 hardness, resistance to wear, and resistanceto deforma tion without an appreciable effect upon their markingqualities.

The cathode electrode may be made of any suitable conductive metal ableto withstand the abrasive effects of the moving recording paper. Metalsfound acceptable are platinum, platinum alloys, steel, and others asknown in the art.

Recording paper may be prepared by impregnating porous white paper orother porous support of high wet strength with an impregnant prepared inaccordance with the teaching set out herein. The impregnant is permittedto distribute itself evenly throughout the paper. Excess impregnant maybe removed by passing the paper between pressure rollers, so that whenready for use in a recorder, the impregnated paper will have about 25 to50 percent moisture by weight. In order to retard evaporation, theimpregnated paper is stored in suitable sealed containers until needed.

While the invention has been described and illustrated with reference tospecific embodiments thereof, it is to be understood that otherembodiments may be resorted to without departing from the invention.Therefore, the form of the invention set out above should be consideredas illustrative and not as limiting the scope of the following claims.

We claim:

1. An electrolytic recording medium comprising an impregnated sheetcontaining in an electrolytically conducting solution a formaldehydesulfoxylate selected from the class consisting of calcium and strontiumformaldehyde sulfoxylate.

2. An electrolytic recording medium according to claim 1 in which theimpregnant contains an electrolyte selected from the group consisting ofalkali metal nitrates, ammonium nitrate, calcium nitrate, and strontiumnitrate.

3. An electrolytic recording medium according to claim 1 in which the pHof the impregnant ranges from 7 to 14.

4. An electrolytic recording medium according to claim 1 in which theimpregnant consists essentially in approximate parts by relative weightof reducing agent 70, electrolyte 100, and water 1000.

5. An electrolytic recording medium according to claim 1 in which theimpregnant is an aqueous solution containing an electrolyte, and calciumformaldehyde sulfoxylate.

6. An electrolytic recording medium according to claim 1 in which theimpregnant is an aqueous solution containing an electrolyte, andstrontium formaldehyde sulfoxylate.

7. An electrolytic recording medium according to claim 1 in which theimpregnant is a solution including in approximate parts by relativeweight water 1000, sodium nitrate 110, calcium hydroxide 2, and calciumformaldehyde sulfoxylate 75.

8. An electrolytic recording medium according to claim 1 in which theimpregnant is a solution including in approximate parts by relativeweight water 1000, strontium nitrate 95, strontium hydroxide 1, andstrontium form aldehyde sulfoxylate 69.

References Cited UNITED STATES PATENTS 12/1936 Elsey 204-2 10/1967Barnes 2042

